1. Field of the Invention
The present invention is directed to a method of making optical replicas by stamping in liquid photoresist and the replicas formed thereby. The stamped photoresist may be used to further transfer the pattern into the substrate through photolithographic processes.
2. Description of Related Art
Replication is a key factor to achieving affordable, mass produced optical elements. Replication is particularly of interest for micro-optical elements and for diffractive optical elements having multiple discrete levels, for which many masks are needed to create such elements photolithographically.
One known replication method is disclosed in U.S. Pat. No. 5,279,689 entitled "Method for Replicating Holographic Elements," which is hereby incorporated by reference in its entirety. In the '689 patent, a layer of embossable material is formed on a substrate. Then a stamper is positioned on the layer and pressed into the surface of the layer. The layer is then cured and the stamper removed, thereby forming the holographic optical element. However, this layer is subject to shifting on and peeling from the substrate.
An attempt to address the problems of the '689 patent is disclosed in U.S. Pat. No. 5,575,878 entitled "Method for Making Surface Relief Profilers," which is hereby incorporated by reference in its entirety. In the '878 patent, a dry photopolymer layer is applied to a glass substrate having a thin layer of polymethylmethacrylate (PMMA) thereon. The dry photopolymer layer is then stamped with a glass master to create the desired pattern therein. The dry photopolymer layer is then cured and then the pattern therein is transferred to the glass substrate by dry etching. In the '878 patent, the PMMA layer is used as an adhesion promoter on the glass substrate so that the dry photopolymer layer will adhere to the substrate, while allowing the glass master to be lifted therefrom.
However, several problems still exist with either method. First, the photopolymer disclosed in the '878 patent degrades during etching, particularly during the faster etching processes, most likely at least in part due to high temperatures in the etching chamber. Therefore, the pattern may not be accurately transferred to the substrate. When trying to create multiple elements simultaneously on a wafer level, while the PMMA helps the dry photopolymer layer adhere, due to the increased surface area of the wafer, part of the dry photopolymer layer may still be pulled away from the substrate, resulting in inaccurate transfer of the pattern. If the photopolymer layer itself was to serve as the relief structure on either a glass or a plastic substrate, as in the '689 patent, even with the PMMA layer, when trying to create multiple elements simultaneously on a wafer level, the stresses resulting when dicing the wafer result in the photopolymer layer peeling or shifting from the substrate even during creation. Finally, the use of the dry photopolymer requires high pressures to be applied in order to transfer the pattern into the dry photopolymer.